A sample custom functor

MyFunctor.js

const add1 = (a) => a + 1;
class MyFunctor { //Custom "Functor"
  constructor(value) {
    this.val = value;
  }
  map(fn) {   //Applies function to this.val + returns new Myfunctor
   return new Myfunctor(fn(this.val));
  }
}
//temp is a Functor instance that's storing value 1
let temp = new MyFunctor(1); 
temp.map(add1) //-> temp allows us to map "add1"

return new Myfunctor(fn(this.val));

Myfunctor
to
MyFunctor